Bacterial infections are currently treated by administration of antibiotics to the infected patient. Antibiotics slow bacterial growth or kill bacteria by various mechanisms including disruption of cell membranes, inhibition of bacterial cell wall synthesis, inhibition of bacterial nucleic acid synthesis, inhibition of bacterial protein synthesis and inhibition of enzymes necessary for bacterial metabolism. In general, antibiotics help decrease the level of infection to a certain threshold, allowing the host's immune system to adjust and help clear the infection. There are drawbacks to using antibiotics. Younger and older patients may be more vulnerable to the toxicity or side effects associated with antibiotics. It is also possible for patients to have or develop allergies to antibiotics. Some antibiotics are also toxic to the patient's helpful, natural flora, which results in upset stomach, diarrhea, etc., which may leave the patient susceptible to new or secondary infections that develop while treating the primary infection. The over-prescription of antibiotics has resulted in many strains of bacteria developing resistance to antibiotics.
Penicillin was the first antibiotic to be identified and used successfully to treat infections in humans. Penicillin is rarely used to treat infections with certain types of bacteria due to widespread resistance of the bacteria to penicillin. New antibiotics have been developed as bacteria become resistant to the current antibiotic being used. Methicillin is an effective antibiotic to treat infections with E. faecium and S. aureus. Methicillin resistance is widespread and most methicillin-resistant strains are also resistant to multiple antibiotics. The term MRSA refers to Methicillin resistant Staphylococcus aureus. Typically, infections with resistant strains of bacteria are first detected in hospital settings and then lead to non-hospital or community-acquired infections. Vancomycin is the antibiotic of last resort to treat infections by methicillin-resistant bacterial strains, and the only antibiotic uniformly effective against MRSA and other methicillin-resistant microbes. However, infections with vancomycin resistant strains of enterococci and S. aureus have been detected in hospitals and are increasing in frequency in community-acquired infections. There is a need to develop a new treatment modality to treat bacterial infections.
Many different types of organisms produce cationic antimicrobial peptides, typically 20-40 amino acids in length, for defense against infection. Most are capable of rapidly killing a wide range of microbial cells. The initial interactions of cationic peptides with Gram-negative bacteria are thought to involve binding to surface lipopolysaccharide and consequently distort the outer membrane bilayer. This allows access to the cytoplasmic membrane where peptide channel formation has been proposed to occur. It is increasingly disputed as to whether peptide channel formation leads to dissolution of the proton motive force and leakage of essential molecules or whether it is an intermediate step in the uptake of peptide into the cytoplasm, where it inhibits an essential function by e.g. binding to polyanionic DNA. However, severe life threatening infections still occur, indicating that virulent bacteria have developed methods to circumvent the innate cationic antimicrobial peptides.
Exemplary antimicrobial peptides include, but are not limited to, cecropins, normally made by lepidoptera (Steiner et al., Nature 292:246, 1981) and diptera (Merrifield et al., Ciba Found. Symp. 186:5, 1994), by porcine intestine (Lee et al., Proc. Nat'l Acad. Sci. USA 86:9159, 1989), by blood cells of a marine protochordate (Zhao et al., FEBS Lett. 412:144, 1997); synthetic analogs of cecropin A, melittin, and cecropin-melittin chimeric peptides (Wade et al., Int. J. Pept. Protein Res. 40:429, 1992); cecropin B analogs (Jaynes et al., Plant Sci. 89:43, 1993); chimeric cecropin A/B hybrids (During, Mol. Breed. 2:297, 1996); magainins (Zasloff, Proc. Nat'l Acad. Sci. USA 84:5449, 1987); cathelin-associated antimicrobial peptides from leukocytes of humans, cattle, pigs, mice, rabbits, and sheep (Zanetti et al., FEBS Lett. 374:1, 1995); vertebrate defensins, such as human neutrophil defensins [HNP 1-4]; paneth cell defensins of mouse and human small intestine (Oulette and Selsted, FASEB J. 10:1280, 1996; Porter et al., Infect. Immun. 65:2396, 1997); vertebrate .beta.-defensins, such as HBD-1 of human epithelial cells (Zhao et al., FEBS Lett. 368:331, 1995); HBD-2 of inflamed human skin (Harder et al., Nature 387:861, 1997); bovine beta.-defensins (Russell et al., Inject. Immun. 64:1565, 1996); plant defensins, such as Rs-AFP1 of radish seeds (Fehlbaum et al., J. Biol. Chem. 269.33159, 1994); alpha.- and beta.-thionins (Stuart et al., Cereal Chem. 19:288, 1942; Bohlmann and Apel, Annu. Rev. Physiol. Plant Mol. Biol. 42:227, 1991); .gamma.-thionins (Broekaert et al., Plant Physiol. 108:1353, 1995); the anti-fungal drosomycin (Fehlbaum et al., J. Biol. Chem. 269:33159, 1994); apidaecins, produced by honey bee, bumble bee, cicada killer, hornet, yellow jacket, and wasp (Casteels et al., J. Biol. Chem. 269:26107, 1994; Levashina et al., Eur. J. Biochem. 233:694, 1995); cathelicidins, such as indolicidin and derivatives or analogues thereof from bovine neutrophils (Falla et al., J. Biol. Chem. 277:19298, 1996); bacteriocins, such as nisin (Delves-Broughton et al., Antonie van Leeuwenhoek J. Microbiol. 69:193, 1996); and the protegrins and tachyplesins, which have antifungal, antibacterial, and antiviral activities (Tamamura et al, Biochim. Biophys. Acta 1163:209, 1993; Aumelas et al., Eur. J. Biochem. 237:575, 1996; Iwanga et al., Ciba Found. Symp. 186:160, 1994). An alternative to treating bacterial infections with antibiotics is to block or inhibit bacterial virulence factors that promote and potentiate infections. However, bacteria produce a wide variety of virulence factors that have many different effects on a host. As such, blocking or inhibiting only one virulence factor is likely to have only a marginal effect on an infection. For example, S. aureus expresses many virulence factors that are grouped as: (1) surface proteins that promote colonization of host tissues; (2) invasins that promote bacterial spread in tissues (e.g. leukocidin, kinases, hyaluronidase); (3) surface factors that inhibit phagocytic engulfment (e.g. capsule, Protein A); (4) biochemical properties that enhance bacterial survival within phagocytes (e.g. carotenoids, catalase production); (5) immunological disguises (e.g. Protein A, coagulase, clotting factor); (6) membrane-damaging toxins that lyse eukaryotic cell membranes (e.g. hemolysins, leukotoxin, leukocidin; (7) exotoxins or enterotoxins that damage host tissues or otherwise provoke symptoms of disease (e.g. SEA-G, TSST, ET) and (8) inherent and acquired resistance to antimicrobial agents. Such staphylococci virulence factors promote the invasion of host tissues and avoidance of host defenses by methods that include the killing of host immune cells and the generation of superantigens that non-specifically overstimulate the host immune system thereby inhibiting a coordinated response against the pathogens by the immune system. For the majority of diseases caused by S. aureus, pathogenesis is multifactorial, so it is difficult to determine precisely the role of any given factor or combination of factors.
It is not clear which of the virulence factors are important for which bacteria, and it is not clear if a virulence factor that has been identified as important in one bacterial species is present and also important in another species.